1. Field of the Invention
The present invention relates in general to control of an air-fuel ratio sensor or oxygen sensor of an all mode or range type, i.e., an air-fuel ratio sensor or oxygen sensor of the type whose output varies smoothly and continuously in response to a variation of the air-fuel ratio from a lean mixture mode or range to a rich mixture mode or range, i.e., throughout all air-fuel ratio mixture modes or ranges, and more particularly to a temperature control method and apparatus for such a type of air-fuel ratio sensor or oxygen sensor.
2. Description of the Related Art
For controlling an air-fuel ratio mixture to be supplied to an engine in a way as to allow the air-fuel ratio to be maintained at a target value (i.e., stoichiometric) and thereby reducing the content of CO, NOx and HC in the engine exhaust gases, it is known to carry out a feedback control of a quantity of fuel to be supplied to the engine. For such feed-back control, a .lambda. (lambda) sensor whose output changes abruptly or sharply (i.e., stepwise) in response to a particular oxygen concentration, i.e., a theoretical air-fuel ratio mixture and an all range air-fuel ratio sensor or oxygen sensor, whose output changes smoothly (i.e., not stepwise) and continuously in response to a variation of the air-fuel ratio from a lean mixture mode or range to a rich mixture mode or range are mainly used. The all range oxygen sensor is, as mentioned above, capable of detecting the oxygen content in an engine exhaust gas continuously and improving the feedback control accuracy and speed, and is thus used in case the higher-speed and more accurate feedback control is required.
An example of such an all range oxygen sensor is disclosed in U.S. Pat. Nos. 5,174,885 and 5,194,135.
The all range oxygen sensor is provided with two cells made of oxygen ion conductive solid electrolytic bodies, which are disposed so as to oppose to each other with a certain interval or gap (measurement chamber) therebetween. One of the cells is used as a pump cell for pumping out the oxygen from or into the gap between the cells. The other of the cells is used as an electromotive force cell for producing a voltage depending upon a difference in the oxygen content between an oxygen reference chamber and the above described gap. The pump cell is operated so that the output of the electromotive force cell is maintained constant, and the current supplied to the pump cell to this end is measured for use as a value proportional to a measured oxygen content. The principle of operation of such an all range oxygen sensor is disclosed in Japanese patent provisional publication No. 62-148849 which is filed by the same assignee as this application.
In order to operate the all range oxygen sensor, it is necessary to heat the above described pump cell and electromotive force cell above a predetermined temperature and thereby activate the oxygen ion conductive solid electrolytic bodies. For this reason, the all range oxygen sensor is provided with a heater in the place adjacent to the pump cell and the electromotive force cell for heating them.
Currently, it is required to reduce more the harmful components such as CO, NOx, HC, etc. contained in the exhaust gases. For removal of those harmful components, it is necessary to measure the oxygen content in the exhaust gases more accurately and carry out a feedback control of an air-fuel ratio at high speed. In this instance, for increasing the accuracy of the oxygen sensor, it is necessary to maintain the temperature of the oxygen sensor constant. For realizing the constant temperature of the oxygen sensor, a method or means is used for measuring a temperature through measurement of the resistance of the heater and to maintain the temperature of the heater constant on the assumption that the measured temperature is equal to that of the cells.
However, by such a method, it has been impossible to control the temperature of the cells with high accuracy due to the fact that when the exhaust gas temperature is low or the flow rate of the exhaust gases is high there occurs a discrepancy between the cell temperature and the heater temperature.